This invention relates to radio broadcasting, and more particularly, to forward error correction in FM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB) and broadcasting systems utilizing such forward error correction.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM IBOC DAB can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog signal. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. FM IBOC broadcasting systems using a hybrid modulation format have been the subject of several United States patents including patents no. 5,465,396; 5,315,583; 5,278,844 and 5,278,826. In addition a commonly assigned pending patent application for a xe2x80x9cMethod and System for Simultaneously Broadcasting and Receiving Digital and Analog Signals, by D. Kumar and B. Hunsinger, Serial No. 08/274,140, filed July 1994 discloses an FM IBOC DAB system now U.S. Pat. No. 5,956,624.
An orthogonal frequency division multiplex (OFDM) technique has been described for IBOC DAB. OFDM signals consist of orthogonally spaced carriers all modulated at a common symbol rate. The frequency spacing for rectangular pulse symbols (e.g., BPSK, QPSK, 8PSK or QAM) is equal to the symbol rate. For IBOC transmission of FM/DAB signals, a redundant set of OFDM sub-carriers is placed within about 100 kHz to 200 kHz on either side of a coexisting analog FM carrier. The DAB power (upper or lower sideband) is set to about xe2x88x9225 dB relative to the FM signal. The level and spectral occupancy of the DAB signal is set to limit interference to its FM host while providing adequate signal-to-noise ration (SNR) for the DAB sub-carriers. First adjacent signals spaced at +xe2x88x92200 kHz from the FM carrier can corrupt the DAB signal. However, at any particular location within a station""s coverage area, it is unlikely that both first adjacents will significantly interfere with DAB. Therefore the upper and lower DAB sidebands carry the same redundant information such that only one sideband is needed to communicate the information. Inherent advantages of OFDM include robustness in the presence of multipath interference, and tolerance to non-gaussian short term noise or notches due to selective fading.
Forward error correction (FEC) and interleaving improve the reliability of the transmitted digital information over a corrupted channel. See for example, S. Kallel, xe2x80x9cComplementary Punctured Convolution (CPC) Codes and Their Applications,xe2x80x9d IEEE Trans. Comm., Vol. 43, No. 6, pp. 2005-2009, June, 1995. Complementary Pair Convolution (CPC) FEC code techniques were developed for Automatic Repeat Request (ARQ) schemes where retransmissions were coded using complementary codes instead of simply retransmitting the same coded sequence. CPC codes can be constructed according to previously published puncturing techniques, e. g. Y. Yasuda, K. Kashiki, Y. Hirata, xe2x80x9cHigh-Rate Punctured Convolutional Codes for Soft Decision Viterbi Decoding,xe2x80x9d IEEE Trans. Comm., Vol. 32, #3, March 1984; and J. Hagenauer, xe2x80x9cRate-Compatible Punctured Convolutional Codes (RCPC Codes) and Their Applications,xe2x80x9d IEEE Trans. Comm., Vol. 36, No. 4, pp. 389-400, April, 1988.
It is known that the periodic puncturing of bits from a convolutional code using Viterbi decoding is an effective means of creating higher rate convolutional codes. Rate compatible punctured convolutional (RCPC) codes have been conceived as a mechanism to adjust coding gain and bit energy as a function of channel capacity in a practical efficient manner, see the above Hagenauer reference or M. Kim, xe2x80x9cOn Systematic Punctured Convolutional Codes,xe2x80x9d IEEE Trans. Comm., Vol 45, No. 2, pp. 133-139, February 1997. This is useful in a point-to-point (non-broadcast) automatic repeat request (ARQ) system where an intended receiver assesses its signal to noise power ratio (Eb/No) and communicates its desire to the transmitter (via a return path) to increase or decrease energy per bit (Eb) and coding gain. The transmitter responds by adjusting its code rate R. This is accomplished with a punctured convolutional code where the transmission of all the bits typically employs an xe2x80x9cindustry standardxe2x80x9d K=7, R=xc2xd rate code, for example. It is assumed in this non-punctured case that the maximum Eb and coding gain is achieved. To improve spectral and/or power efficiency, the transmitter may elect to eliminate (puncture at the receiver""s request, for example) the transmission of some of the coded bits, resulting in a higher rate code. This puncturing has the effect of lowering the effective Eb and coding gain relative to the original unpunctured code; however, this punctured code may still be sufficient to successfully communicate the information over the channel in a more efficient manner.
For best performance at a given code rate, a particular pattern of bits in the coded sequence is punctured. Unfortunately, the puncture pattern for higher rate codes does not include all the bits punctured for lower rate codes. Haganauer showed that the puncture patterns for his RCPC codes can include all punctures for lower rate codes with little loss compared to the optimal, but rate-incompatible, puncture patterns. Therefore the code rate can be increased from the original R=xc2xd code simply by puncturing more of the puncturable bits of the same pattern. The higher rate codes are a subset of the bits of the lower rate codes.
The interference environment in VHF FM-band IBOC DAB channel is generally such that a DAB channel can be dichotomized into the following two subsets of subchannels: (a) a reliable part composed of regions of spectrum relatively free of interference from other stations""signals, characterized as being thermal or background noise limited, with multipath fading as an impairment; and (b) an unreliable part composed of regions of spectrum with intermittent intervals of heavy interference which corrupts the bits transmitted during those intervals, but is at other times (or for most geographical locations) similar to the reliable part described above. AM band IBOC DAB can be similarly characterized.
The prior art utilizes one of two fundamental strategies to transmit data in this environment: (1) simply do not utilize the unreliable part of the channel, thus those times during which the unreliable part is clear and usable are essentially wasted; or (2) utilize a sufficiently low rate code (and appropriately increased coded bit rate) to guarantee the required bit error rate (BER), and spread the increased bandwidth across both the reliable and unreliable parts of the spectrum evenly. This is done by uniform allocation of bits to OFDM carriers in an OFDM system, or increasing the raw bit rate of a single carrier system. This utilizes the unreliable part of the channel, but also incurs a BER penalty (possibly catastrophic) when severe interference occurs in the unreliable part of the channel. Depending on the interference, the second alternative may or may not be better than the first.
This invention addresses non-uniform interference through special coding and error handling to achieve more robust performance. The broadcasting method of the invention encodes program material using convolutional codes having non-puncturable bits and puncturable bits and modulates orthogonal frequency division multiplexed carrier signals with the convolutional codes. The non-puncturable bits are carried by a first group of the carriers and the puncturable bits are carried by a second group of the carriers, where he first group of carrier signals is less susceptible to interference than the second group of carrier signals. The carrier signals are then broadcast to receivers which determine if the carriers in the second group have been corrupted and erase puncturable bits carried by any of the carriers which have been determined to be corrupted. This produces punctured codes which are subsequently decoded to recover the program material.
The invention also encompasses transmitters and receivers which operate in accordance with the method of the invention. This invention provides an FEC coding technique that results in an improved bit error rate in the interference environment of an in-band-on-channel digital audio broadcasting system using orthogonal frequency division multiplexing. This mitigates the effects of interference from non-uniform interference in orthogonal frequency division multiplex broadcast systems.